RFC 5225
RObust Header Compression Version 2 (ROHCv2): Profiles for RTP, UDP, IP, ESP and UDP-Lite
6. ROHCv2 Profiles (Normative)
6.1. Channel Parameters, Segmentation, and Reordering
The compressor MUST NOT use ROHC segmentation (see Section 5.2.5 of [RFC4995]), i.e., the Maximum Reconstructed Reception Unit (MRRU) MUST be set to 0, if the configuration of the ROHC channel contains at least one ROHCv2 profile in the list of supported profiles (i.e., the PROFILES parameter) and if the channel cannot guarantee in-order delivery of packets between compression endpoints.
6.2. Profile Operation, Per-context
ROHCv2 profiles operate differently, per context, depending on how the decompressor makes use of the feedback channel, if any. Once the decompressor uses the feedback channel for a context, it establishes the feedback channel for that CID. The compressor always starts with the assumption that the decompressor will not send feedback when it initializes a new context (see also the definition of a new context in Section 5.1.1. of [RFC4995], i.e., there is no established feedback channel for the new context. At this point, despite the use of the optimistic approach, decompression failure is still possible because the decompressor may not have received sufficient information to correctly decompress the packets; therefore, until the decompressor has established a feedback channel, the compressor SHOULD periodically send IR packets. The periodicity can be based on timeouts, on the number of compressed packets sent for the flow, or any other strategy the implementer chooses. The reception of either positive feedback (ACKs) or negative feedback (NACKs or STATIC-NACKs) from the decompressor establishes the feedback channel for the context (CID) for which the feedback was received. Once there is an established feedback channel for a specific context, the compressor can make use of this feedback to estimate the current state of the decompressor. This helps to increase the compression efficiency by providing the information needed for the compressor to achieve the necessary confidence level. When the feedback channel is established, it becomes superfluous for the compressor to send periodic refreshes, and instead it can rely entirely on the optimistic approach and feedback from the decompressor. The decompressor MAY send positive feedback (ACKs) to initially establish the feedback channel for a particular flow. Either positive feedback (ACKs) or negative feedback (NACKs or STATIC-NACKs) establishes this channel. Once it has established a feedback channel for a CID, the decompressor is REQUIRED to continue sending feedback for the lifetime of the context (i.e., until it receives an IR packet that associates the CID to a different profile), to send error recovery requests and (optionally) acknowledgments of significant context updates. Compression without an established feedback channel will be less efficient, because of the periodic refreshes and the lack of feedback to trigger error recovery; there will also be a slightly higher probability of loss propagation compared to the case where the decompressor uses feedback.
6.3. Control Fields
ROHCv2 defines a number of control fields that are used by the decompressor in its interpretation of the header formats received from the compressor. The control fields listed in the following subsections are defined using the formal notation [RFC4997] in Section 6.8.2.4 of this document.6.3.1. Master Sequence Number (MSN)
The Master Sequence Number (MSN) field is either taken from a field that already exists in one of the headers of the protocol that the profile compresses (e.g., RTP SN), or alternatively it is created at the compressor. There is one MSN space per context. The MSN field has the following two functions: o Differentiating between reference headers when receiving feedback data; o Inferring the value of incrementing fields (e.g., IPv4 Identifier). There is one MSN field in every ROHCv2 header, i.e., the MSN is always present in each header type sent by the compressor. The MSN is sent in full in IR headers, while it can be lsb encoded within CO header formats. The decompressor always includes LSBs of the MSN in the Acknowledgment Number field in feedback (see Section 6.9). The compressor can later use this field to infer what packet the decompressor is acknowledging. For profiles for which the MSN is created by the compressor (i.e., 0x0102, 0x0104, and 0x0108), the following applies: o The compressor only initializes the MSN for a context when that context is first created or when the profile associated with a context changes; o When the MSN is initialized, it is initialized to a random value; o The value of the MSN SHOULD be incremented by one for each packet that the compressor sends for a specific CID.6.3.2. Reordering Ratio
The control field reorder_ratio specifies how much reordering is handled by the lsb encoding of the MSN. This is useful when header compression is performed over links with varying reordering
characteristics. The reorder_ratio control field provides the means for the compressor to adjust the robustness characteristics of the lsb encoding method with respect to reordering and consecutive losses, as described in Section 5.1.2.6.3.3. IP-ID Behavior
The IP-ID field of the IPv4 header can have different change patterns: sequential in network byte order, sequential byte-swapped, random or constant (a constant value of zero, although not conformant with [RFC0791], has been observed in practice). There is one IP-ID behavior control field per IP header. The control field for the IP-ID behavior of the innermost IP header determines which set of header formats is used. The IP-ID behavior control field is also used to determine the contents of the irregular chain item, for each IP header. ROHCv2 profiles MUST NOT assign a sequential behavior (network byte order or byte-swapped) to any IP-ID but the one in the innermost IP header when compressing more than one level of IP headers. This is because only the IP-ID of the innermost IP header is likely to have a sufficiently close correlation with the MSN to compress it as a sequentially changing field. Therefore, a compressor MUST assign either the constant zero IP-ID or the random IP-ID behavior to tunneling headers.6.3.4. UDP-Lite Coverage Behavior
The control field coverage_behavior specifies how the checksum coverage field of the UDP-Lite header is compressed with RoHCv2. It can indicate one of the following encoding methods: irregular, static, or inferred encoding.6.3.5. Timestamp Stride
The ts_stride control field is used in scaled RTP timestamp encoding (see Section 6.6.8). It defines the expected increase in the RTP timestamp between consecutive RTP sequence numbers.6.3.6. Time Stride
The time_stride control field is used in timer-based compression encoding (see Section 6.6.9). When timer-based compression is used, time_stride should be set to the expected difference in arrival time between consecutive RTP packets.
6.3.7. CRC-3 for Control Fields
ROHCv2 profiles define a CRC-3 calculated over a number of control fields. This 3-bit CRC protecting the control fields is present in the header format for the co_common and co_repair header types. The decompressor MUST always validate the integrity of the control fields covered by this 3-bit CRC when processing a co_common or a co_repair compressed header. Failure to validate the control fields using this CRC should be considered as a decompression failure by the decompressor in the algorithm that assesses the validity of the context. However, if the decompression attempt can be verified using either the CRC-3 or the CRC-7 calculated over the uncompressed header, the decompressor MAY still forward the decompressed header to upper layers. This is because the protected control fields are not always used to decompress the header (i.e., co_common or co_repair) that updates their respective value. The CRC polynomial and coverage of this CRC-3 is defined in Section 6.6.11.6.4. Reconstruction and Verification
Validation of the IR header (8-bit CRC) The decompressor MUST always validate the integrity of the IR header using the 8-bit CRC carried within the IR header. When the header is validated, the decompressor updates the context with the information in the IR header. Otherwise, if the IR cannot be validated, the context MUST NOT be updated and the IR header MUST NOT be delivered to upper layers. Verification of CO headers (3-bit CRC or 7-bit CRC) The decompressor MUST always verify the decompression of a CO header using the CRC carried within the compressed header. When the decompression is verified and successful, the decompressor updates the context with the information received in the CO header; otherwise, if the reconstructed header fails the CRC verification, these updates MUST NOT be performed. A packet for which the decompression attempt cannot be verified using the CRC MUST NOT be delivered to upper layers.
Decompressor implementations may attempt corrective or repair
measures on CO headers prior to performing the above actions, and
the result of any decompression attempt MUST be verified using the
CRC.
6.5. Compressed Header Chains
Some header types use one or more chains containing sub-header
information. The function of a chain is to group fields based on
similar characteristics, such as static, dynamic, or irregular
fields.
Chaining is done by appending an item for each header to the chain in
their order of appearance in the uncompressed packet, starting from
the fields in the outermost header.
In the text below, the term <protocol_name> is used to identify
formal notation names corresponding to different protocol headers.
The mapping between these is defined in the following table:
+----------------------------------+---------------+
| Protocol | protocol_name |
+----------------------------------+---------------+
| IPv4 RFC 0791 | ipv4 |
| IPv6 RFC 2460 | ipv6 |
| UDP RFC 0768 | udp |
| RTP RFC 3550 | rtp |
| ESP RFC 4303 | esp |
| UDP-Lite RFC 3828 | udp_lite |
| AH RFC 4302 | ah |
| GRE RFC 2784, RFC 2890 | gre |
| MINE RFC 2004 | mine |
| IPv6 Destination Option RFC 2460 | dest_opt |
| IPv6 Hop-by-hop Options RFC 2460 | hop_opt |
| IPv6 Routing Header RFC 2460 | rout_opt |
+----------------------------------+---------------+
Static chain:
The static chain consists of one item for each header of the chain
of protocol headers that is compressed, starting from the
outermost IP header. In the formal description of the header
formats, this static chain item for each header type is labeled
<protocol_name>_static. The static chain is only used in the IR
header format.
Dynamic chain:
The dynamic chain consists of one item for each header of the
chain of protocol headers that is compressed, starting from the
outermost IP header. In the formal description of the header
formats, the dynamic chain item for each header type is labeled
<protocol_name>_dynamic. The dynamic chain is only used in the IR
and co_repair header formats.
Irregular chain:
The structure of the irregular chain is analogous to the structure
of the static chain. For each compressed header that uses the
general format of Section 6.8, the irregular chain is appended at
a specific location in the general format of the compressed
headers. In the formal description of the header formats, the
irregular chain item for each header type is a format whose name
is suffixed by "_irregular". The irregular chain is used in all
CO headers, except for the co_repair format.
The format of the irregular chain for the innermost IP header
differs from the format used for the outer IP headers, because the
innermost IP header is part of the compressed base header. In the
definition of the header formats using the formal notation, the
argument "is_innermost", which is passed to the corresponding
encoding method (ipv4 or ipv6), determines what irregular chain
items to use. The format of the irregular chain item for the
outer IP headers is also determined using one flag for TTL/Hop
Limit and TOS/TC. This flag is defined in the format of some of
the compressed base headers.
ROHCv2 profiles compress extension headers as other headers, and thus
extension headers have a static chain, a dynamic chain, and an
irregular chain.
ROHCv2 profiles define chains for all headers that can be compressed,
i.e., RTP [RFC3550], UDP [RFC0768], ESP [RFC4303], UDP-Lite
[RFC3828], IPv4 [RFC0791], IPv6 [RFC2460], AH [RFC4302], GRE
[RFC2784][RFC2890], MINE [RFC2004], IPv6 Destination Options header
[RFC2460], IPv6 Hop-by-hop Options header [RFC2460], and IPv6 Routing
header [RFC2460].
6.6. Header Formats and Encoding Methods
The header formats are defined using the ROHC formal notation. Some
of the encoding methods used in the header formats are defined in
[RFC4997], while other methods are defined in this section.
6.6.1. baseheader_extension_headers
The baseheader_extension_headers encoding method skips over all fields of the extension headers of the innermost IP header, without encoding any of them. Fields in these extension headers are instead encoded in the irregular chain. This encoding is used in CO headers (see Section 6.8.2). The innermost IP header is combined with other header(s) (i.e., UDP, UDP- Lite, RTP) to create the compressed base header. In this case, there may be a number of extension headers between the IP headers and the other headers. The base header defines a representation of the extension headers, to comply with the syntax of the formal notation; this encoding method provides this representation.6.6.2. baseheader_outer_headers
The baseheader_outer_headers encoding method skips over all the fields of the extension header(s) that do not belong to the innermost IP header, without encoding any of them. Changing fields in outer headers are instead handled by the irregular chain. This encoding method, similarly to the baseheader_extension_headers encoding method above, is necessary to keep the definition of the header formats syntactically correct. It describes tunneling IP headers and their respective extension headers (i.e., all headers located before the innermost IP header) for CO headers (see Section 6.8.2).6.6.3. inferred_udp_length
The decompressor infers the value of the UDP length field as being the sum of the UDP header length and the UDP payload length. The compressor must therefore ensure that the UDP length field is consistent with the length field(s) of preceding subheaders, i.e., there must not be any padding after the UDP payload that is covered by the IP Length. This encoding method is also used for the UDP-Lite Checksum Coverage field when it behaves in the same manner as the UDP length field (i.e., when the checksum always covers the entire UDP-Lite payload).6.6.4. inferred_ip_v4_header_checksum
This encoding method compresses the header checksum field of the IPv4 header. This checksum is defined in RFC 791 [RFC0791] as follows:
Header Checksum: 16 bits
A checksum on the header only. Since some header fields change
(e.g., time to live), this is recomputed and verified at each
point that the internet header is processed.
The checksum algorithm is:
The checksum field is the 16 bit one's complement of the one's
complement sum of all 16 bit words in the header. For purposes
of computing the checksum, the value of the checksum field is
zero.
As described above, the header checksum protects individual hops from
processing a corrupted header. As the data that this checksum
protects is mostly compressed away and is instead taken from state
stored in the context, this checksum becomes cumulative to the ROHC
CRC. When using this encoding method, the checksum is recomputed by
the decompressor.
The inferred_ip_v4_header_checksum encoding method thus compresses
the header checksum field of the IPv4 header down to a size of zero
bits, i.e., no bits are transmitted in compressed headers for this
field. Using this encoding method, the decompressor infers the value
of this field using the computation above.
The compressor MAY use the header checksum to validate the
correctness of the header before compressing it, to avoid processing
a corrupted header.
6.6.5. inferred_mine_header_checksum
This encoding method compresses the minimal encapsulation header
checksum. This checksum is defined in RFC 2004 [RFC2004] as follows:
Header Checksum
The 16-bit one's complement of the one's complement sum of all
16-bit words in the minimal forwarding header. For purposes of
computing the checksum, the value of the checksum field is 0.
The IP header and IP payload (after the minimal forwarding
header) are not included in this checksum computation.
The inferred_mine_header_checksum encoding method compresses the
minimal encapsulation header checksum down to a size of zero bits,
i.e., no bits are transmitted in compressed headers for this field.
Using this encoding method, the decompressor infers the value of this
field using the above computation.
The motivations for inferring this checksum are similar to the ones explained above in Section 6.6.4. The compressor MAY use the minimal encapsulation header checksum to validate the correctness of the header before compressing it, to avoid processing a corrupted header.6.6.6. inferred_ip_v4_length
This encoding method compresses the total length field of the IPv4 header. The total length field of the IPv4 header is defined in RFC 791 [RFC0791] as follows: Total Length: 16 bits Total Length is the length of the datagram, measured in octets, including internet header and data. This field allows the length of a datagram to be up to 65,535 octets. The inferred_ip_v4_length encoding method compresses the IPv4 header checksum down to a size of zero bits, i.e., no bits are transmitted in compressed headers for this field. Using this encoding method, the decompressor infers the value of this field by counting in octets the length of the entire packet after decompression.6.6.7. inferred_ip_v6_length
This encoding method compresses the payload length field in the IPv6 header. This length field is defined in RFC 2460 [RFC2460] as follows: Payload Length: 16-bit unsigned integer Length of the IPv6 payload, i.e., the rest of the packet following this IPv6 header, in octets. (Note that any extension headers present are considered part of the payload, i.e., included in the length count.) The "inferred_ip_v6_length" encoding method compresses the payload length field of the IPv6 header down to a size of zero bits, i.e., no bits are transmitted in compressed headers for this field. Using this encoding method, the decompressor infers the value of this field by counting in octets the length of the entire packet after decompression. IPv6 headers using the jumbo payload option of RFC 2675 [RFC2675] will not be compressible with this encoding method since the value of the payload length field does not match the length of the packet.
6.6.8. Scaled RTP Timestamp Compression
This section provides additional details on encodings used to scale the RTP timestamp, as defined in the formal notation in Section 6.8.2.4. The RTP timestamp (TS) usually increases by a multiple of the RTP Sequence Number's (SN's) increase and is therefore a suitable candidate for scaled encoding. This scaling factor is labeled ts_stride in the definition of the profile in the formal notation. The compressor sets the scaling factor based on the change in TS with respect to the change in the RTP SN. The default value of the scaling factor ts_stride is 160, as defined in Section 6.8.2.4. To use a different value for ts_stride, the compressor explicitly updates the value of ts_stride to the decompressor using one of the header formats that can carry this information. When the compressor uses a scaling factor that is different than the default value of ts_stride, it can only use the new scaling factor once it has enough confidence that the decompressor has successfully calculated the residue (ts_offset) of the scaling function for the timestamp. The compressor achieves this by sending unscaled timestamp values, to allow the decompressor to establish the residue based on the current ts_stride. The compressor MAY send the unscaled timestamp in the same compressed header(s) used to establish the value of ts_stride. Once the compressor has gained enough confidence that both the value of the scaling factor and the value of the residue have been established in the decompressor, the compressor can start compressing packets using the new scaling factor. When the compressor detects that the residue (ts_offset) value has changed, it MUST NOT select a compressed header format that uses the scaled timestamp encoding before it has re-established the residue as described above. When the value of the timestamp field wraps around, the value of the residue of the scaling function is likely to change. When this occurs, the compressor re-establishes the new residue value as described above. If the decompressor receives a compressed header containing scaled timestamp bits while the ts_stride equals zero, it MUST NOT deliver the packet to upper layers and it SHOULD treat this as a CRC verification failure.
Whether or not the scaling is applied to the RTP TS field is up to the compressor implementation (i.e., the use of scaling is OPTIONAL), and is indicated by the tsc_indicator control field. In case scaling is applied to the RTP TS field, the value of ts_stride used by the compressor is up to the implementation. A value of ts_stride that is set to the expected increase in the RTP timestamp between consecutive unit increases of the RTP SN will provide the most gain for the scaled encoding. Other values may provide the same gain in some situations, but may reduce the gain in others. When scaled timestamp encoding is used for header formats that do not transmit any lsb-encoded timestamp bits at all, the inferred_scaled_field encoding of Section 6.6.10 is used for encoding the timestamp.6.6.9. timer_based_lsb
The timer-based compression encoding method, timer_based_lsb, compresses a field whose change pattern approximates a linear function of the time of day. This encoding uses the local clock to obtain an approximation of the value that it encodes. The approximated value is then used as a reference value together with the num_lsbs_param least-significant bits received as the encoded value, where num_lsbs_param represents a number of bits that is sufficient to uniquely represent the encoded value in the presence of jitter between compression endpoints. ts_scaled =:= timer_based_lsb(<time_stride_param>, <num_lsbs_param>, <offset_param>) The parameters "num_lsbs_param" and "offset_param" are the parameters to use for the lsb encoding, i.e., the number of least significant bits and the interpretation interval offset, respectively. The parameter "time_stride_param" represents the context value of the control field time_stride. This encoding method always uses a scaled version of the field it compresses. The value of the field is decoded by calculating an approximation of the scaled value, using: tsc_ref_advanced = tsc_ref + (a_n - a_ref) / time_stride.
where:
- tsc_ref is a reference value of the scaled representation
of the field.
- a_n is the arrival time associated with the value to decode.
- a_ref is the arrival time associated with the reference header.
- tsc_ref_advanced is an approximation of the scaled value
of the field.
The lsb encoding is then applied using the num_lsbs_param bits
received in the compressed header and the tsc_ref_advanced as
"ref_value" (as per Section 4.11.5 of [RFC4997]).
Appendix B.3 provides an example of how the compressor can calculate
jitter.
The control field time_stride controls whether or not the
timer_based_lsb method is used in the CO header. The decompressor
SHOULD send the CLOCK_RESOLUTION option with a zero value, if:
o it receives a non-zero time_stride value, and
o it has not previously sent a CLOCK_RESOLUTION feedback with a non-
zero value.
This is to allow compression to recover from the case where a
compressor erroneously activates timer-based compression.
The support and usage of timer-based compression is OPTIONAL for both
the compressor and the decompressor; the compressor is not required
to set the time_stride control field to a non-zero value when it has
received a non-zero value for the CLOCK_RESOLUTION option.
6.6.10. inferred_scaled_field
The inferred_scaled_field encoding method encodes a field that is
defined as changing in relation to the MSN, and for which the
increase with respect to the MSN can be scaled by some scaling
factor. This encoding method is used in compressed header formats
that do not contain any bits for the scaled field. In this case, the
decompressor infers the unscaled value of the scaled field from the
MSN field. The unscaled value is calculated according to the
following formula:
unscaled_value = delta_msn * stride + reference_unscaled_value
where "delta_msn" is the difference in MSN between the reference
value of the MSN in the context and the value of the MSN decompressed
from this packet, "reference_unscaled_value" is the value of the
field being scaled in the context, and "stride" is the scaling value
for this field.
For example, when this encoding method is applied to the RTP
timestamp in the RTP profile, the calculation above becomes:
timestamp = delta_msn * ts_stride + reference_timestamp
6.6.11. control_crc3_encoding
The control_crc3_encoding method provides a CRC calculated over a
number of control fields. The definition of this encoding method is
the same as for the "crc" encoding method specified in Section 4.11.6
of [RFC4997], with the difference being that the data covered by the
CRC is given by a concatenated list of control fields.
In other words, the definition of the control_crc3_encoding method is
equivalent to the following definition:
control_crc_encoding(ctrl_data_value, ctrl_data_length)
{
UNCOMPRESSED {
}
COMPRESSED {
control_crc3 =:=
crc(3, 0x06, 0x07, ctrl_data_value, ctrl_data_length) [ 3 ];
}
}
where the parameter "ctrl_data_value" binds to the concatenated
values of the following control fields, in the order listed below:
o reorder_ratio, 2 bits padded with 6 MSB of zeroes
o ts_stride, 32 bits (only for profiles 0x0101 and 0x0107)
o time_stride, 32 bits (only for profiles 0x0101 and 0x0107)
o msn, 16 bits (not applicable for profiles 0x0101, 0x0103, and
0x0107)
o coverage_behavior, 2 bits padded with 6 MSB of zeroes (only for
profiles 0x0107 and 0x0108)
o ip_id_behavior, one octet for each IP header in the compressible
header chain starting from the outermost header. Each octet
consists of 2 bits padded with 6 MSBs of zeroes.
The "ctrl_data_length" binds to the sum of the length of the control
field(s) that are applicable to the specific profile.
The decompressor uses the resulting 3-bit CRC to validate the control
fields that are updated by the co_common and co_repair header
formats; this CRC cannot be used to verify the outcome of a
decompression attempt.
This CRC protects the update of control fields, as the updated values
are not always used to decompress the header that carries them and
thus are not protected by the CRC-7 verification. This prevents
impairments that could occur if the decompression of a co_common or
of a co_repair succeeds and the decompressor sends positive feedback,
while for some reason the control fields are incorrectly updated.
6.6.12. inferred_sequential_ip_id
This encoding method is used with a sequential IP-ID behavior
(sequential or sequential byte-swapped) and when there are no coded
IP-ID bits in the compressed header. In this case, the IP-ID offset
from the MSN is constant, and the IP-ID increases by the same amount
as the MSN (similar to the inferred_scaled_field encoding method).
The decompressor calculates the value for the IP-ID according to the
following formula:
IP-ID = delta_msn + reference_IP_ID_value
where "delta_msn" is the difference between the reference value of
the MSN in the context and the uncompressed value of the MSN
associated to the compressed header, and where
"reference_IP_ID_value" is the value of the IP-ID in the context.
For swapped IP-ID behavior (i.e., when ip_id_behavior_innermost is
set to IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED), "reference_IP_ID_value"
and "IP-ID" are byte-swapped with regard to the corresponding fields
in the context.
If the IP-ID behavior is random or zero, this encoding method does
not update any fields.
6.6.13. list_csrc(cc_value)
This encoding method compresses the list of RTP CSRC identifiers using list compression. This encoding establishes a content for the different CSRC identifiers (items) and a list describing the order in which they appear. The compressor passes an argument (cc_value) to this encoding method: this is the value of the CC field taken from the RTP header. The decompressor is required to bind the value of this argument to the number of items in the list, which will allow the decompressor to correctly reconstruct the CC field.6.6.13.1. List Compression
The CSRC identifiers in the uncompressed packet can be represented as an ordered list, whose order and presence are usually constant between packets. The generic structure of such a list is as follows: +--------+--------+--...--+--------+ list: | item 1 | item 2 | | item n | +--------+--------+--...--+--------+ When performing list compression on a CSRC list, each item is the uncompressed value of one CSRC identifier. The basic principles of list-based compression are the following: When initializing the context: 1) The complete representation of the list of CSRC identifiers is transmitted. Then, once the context has been initialized: 2) When the list is unchanged, a compressed header that does not contain information about the list can be used. 3) When the list changes, a compressed list is sent in the compressed header, including a representation of its structure and order. Previously unknown items are sent uncompressed in the list, while previously known items are only represented by an index pointing to the item stored in the context.
6.6.13.2. Table-based Item Compression
The table-based item compression compresses individual items sent in compressed lists. The compressor assigns a unique identifier, "Index", to each item "Item" of a list. Compressor Logic The compressor conceptually maintains an item table containing all items, indexed using "Index". The (Index, Item) pair is sent together in compressed lists until the compressor gains enough confidence that the decompressor has observed the mapping between items and their respective index. Confidence is obtained from the reception of an acknowledgment from the decompressor, or by sending (Index, Item) pairs using the optimistic approach. Once confidence is obtained, the index alone is sent in compressed lists to indicate the presence of the item corresponding to this index. The compressor MAY reset its item table upon receiving a negative acknowledgement. The compressor MAY reassign an existing index to a new item by re- establishing the mapping using the procedure described above. Decompressor Logic The decompressor conceptually maintains an item table that contains all (Index, Item) pairs received. The item table is updated whenever an (Index, Item) pair is received and decompression is successful (CRC verification, or CRC-8 validation). The decompressor retrieves the item from the table whenever an Index is received without an accompanying Item. If an index is received without an accompanying Item and the decompressor does not have any context for this index, the decompressor MUST NOT deliver the packet to upper layers.6.6.13.3. Encoding of Compressed Lists
Each item present in a compressed list is represented by: o an Index into the table of items, and a presence bit indicating if a compressed representation of the item is present in the list. o an item (if the presence bit is set).
If the presence bit is not set, the item must already be known by the
decompressor.
A compressed list of items uses the following encoding:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Reserved |PS | m |
+---+---+---+---+---+---+---+---+
| XI_1, ..., XI_m | m octets, or m * 4 bits
/ --- --- --- ---/
| : Padding : if PS = 0 and m is odd
+---+---+---+---+---+---+---+---+
| |
/ Item_1, ..., Item_n / variable
| |
+---+---+---+---+---+---+---+---+
Reserved: MUST be set to zero; otherwise, the decompressor MUST
discard the packet.
PS: Indicates size of XI fields:
PS = 0 indicates 4-bit XI fields;
PS = 1 indicates 8-bit XI fields.
m: Number of XI item(s) in the compressed list. Also, the value
of the cc_value argument of the list_csrc encoding (see
Section 6.6.13).
XI_1, ..., XI_m: m XI items. Each XI represents one item in the
list of items of the uncompressed header, in the same order as
they appear in the uncompressed header.
The format of an XI item is as follows:
0 1 2 3
+---+---+---+---+
PS = 0: | X | Index |
+---+---+---+---+
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
PS = 1: | X | Reserved | Index |
+---+---+---+---+---+---+---+---+
X: Indicates whether the item is present in the list:
X = 1 indicates that the item corresponding to the Index is
sent in the Item_1, ..., Item_n list;
X = 0 indicates that the item corresponding to the Index is
not sent.
Reserved: MUST be set to zero; otherwise, the decompressor MUST
discard the packet.
Index: An index into the item table. See Section 6.6.13.4
When 4-bit XI items are used, the XI items are placed in octets
in the following manner:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| XI_k | XI_k + 1 |
+---+---+---+---+---+---+---+---+
Padding: A 4-bit Padding field is present when PS = 0 and the
number of XIs is odd. The Padding field MUST be set to zero;
otherwise, the decompressor MUST discard the packet.
Item 1, ..., item n: Each item corresponds to an XI with X = 1 in
XI 1, ..., XI m. Each entry in the Item list is the uncompressed
representation of one CSRC identifier.
6.6.13.4. Item Table Mappings
The item table for list compression is limited to 16 different items,
since the RTP header can only carry at most 15 simultaneous CSRC
identifiers. The effect of having more than 16 items in the item
table will only cause a slight overhead to the compressor when items
are swapped in/out of the item table.
6.6.13.5. Compressed Lists in Dynamic Chain
A compressed list that is part of the dynamic chain must have all of
its list items present, i.e., all X-bits in the XI list MUST be set.
All items previously established in the item table that are not
present in the list decompressed from this packet MUST also be
retained in the decompressor context.
6.7. Encoding Methods with External Parameters as Arguments
A number of encoding methods in Section 6.8.2.4 have one or more arguments for which the derivation of the parameter's value is outside the scope of the ROHC-FN [RFC4997] specification of the header formats. The following is a list of encoding methods with external parameters as arguments, from Section 6.8.2.4: o udp(profile_value, reorder_ratio_value) o udp_lite(profile_value, reorder_ratio_value, coverage_behavior_value) o esp(profile_value, reorder_ratio_value) o rtp(profile_value, ts_stride_value, time_stride_value, reorder_ratio_value) o ipv4(profile_value, is_innermost, outer_ip_flag, ip_id_behavior_value, reorder_ratio_value)) o ipv6(profile_value, is_innermost, outer_ip_flag, reorder_ratio_value)) o iponly_baseheader(profile_value, outer_ip_flag, ip_id_behavior_value, reorder_ratio_value) o udp_baseheader(profile_value, outer_ip_flag, ip_id_behavior_value, reorder_ratio_value) o udplite_baseheader(profile_value, outer_ip_flag, ip_id_behavior_value, reorder_ratio_value) o esp_baseheader(profile_value, outer_ip_flag, ip_id_behavior_value, reorder_ratio_value) o rtp_baseheader(profile_value, ts_stride_value, time_stride_value, outer_ip_flag, ip_id_behavior_value, reorder_ratio_value) o udplite_rtp_baseheader(profile_value, ts_stride_value, time_stride_value, outer_ip_flag, ip_id_behavior_value, reorder_ratio_value, coverage_behavior_value) The following applies for all parameters listed below: At the compressor, the value of the parameter is set according to the recommendations for each parameter. At the decompressor, the value
of the parameter is set to undefined and will get bound by encoding
methods, except where otherwise noted.
The following is a list of external arguments with their respective
definition:
o profile_value:
Set to the 16-bit number that identifies the profile used to
compress this packet. When processing the static chain at the
decompressor, this parameter is set to the value of the profile
field in the IR header (see Section 6.8.1).
o reorder_ratio_value:
Set to a 2-bit integer value, using one of the constants whose
name begins with the prefix REORDERING_ and as defined in
Section 6.8.2.4.
o ip_id_behavior_value:
Set to a 2-bit integer value, using one of the constants whose
name begins with the prefix IP_ID_BEHAVIOR_ and as defined in
Section 6.8.2.4.
o coverage_behavior_value:
Set to a 2-bit integer value, using one of the constants whose
name begins with the prefix UDP_LITE_COVERAGE_ and as defined
in Section 6.8.2.4.
o outer_ip_flag:
This parameter is set to 1 if at least one of the TOS/TC or
TTL/Hop Limit fields in outer IP headers has changed compared
to their reference values in the context; otherwise, it is set
to 0. This flag may only be set to 1 for the "co_common"
header format in the different profiles.
o is_innermost:
This boolean flag is set to 1 when processing the innermost of
the compressible IP headers; otherwise, it is set to 0.
o ts_stride_value
The value of this parameter should be set to the expected
increase in the RTP Timestamp between consecutive RTP sequence
numbers. The value selected is implementation-specific. See
also Section 6.6.8.
o time_stride_value
The value of this parameter should be set to the expected
inter-arrival time between consecutive packets for the flow.
The value selected is implementation-specific. This parameter
MUST be set to zero, unless the compressor has received a
feedback message with the CLOCK_RESOLUTION option set to a non-
zero value. See also Section 6.6.9.
6.8. Header Formats
ROHCv2 profiles use two different header types: the Initialization
and Refresh (IR) header type, and the Compressed header type (CO).
The CO header type defines a number of header formats: there are two
sets of base header formats, with a few additional formats that are
common to both sets.
6.8.1. Initialization and Refresh Header Format (IR)
The IR header format uses the structure of the ROHC IR header as
defined in Section 5.2.2.1 of [RFC4995].
Header type: IR
This header format communicates the static part and the dynamic
part of the context.
The ROHCv2 IR header has the following format:
0 1 2 3 4 5 6 7
--- --- --- --- --- --- --- ---
: Add-CID octet : if for small CIDs and (CID != 0)
+---+---+---+---+---+---+---+---+
| 1 1 1 1 1 1 0 1 | IR type octet
+---+---+---+---+---+---+---+---+
: :
/ 0-2 octets of CID / 1-2 octets if for large CIDs
: :
+---+---+---+---+---+---+---+---+
| Profile | 1 octet
+---+---+---+---+---+---+---+---+
| CRC | 1 octet
+---+---+---+---+---+---+---+---+
| |
/ Static chain / variable length
| |
- - - - - - - - - - - - - - - -
| |
/ Dynamic chain / variable length
| |
- - - - - - - - - - - - - - - -
CRC: 8-bit CRC over the entire IR-header, including any CID fields
and up until the end of the dynamic chain, using the polynomial
defined in [RFC4995]. For purposes of computing the CRC, the CRC
field is zero.
Static chain: See Section 6.5.
Dynamic chain: See Section 6.5.
6.8.2. Compressed Header Formats (CO)
6.8.2.1. Design Rationale for Compressed Base Headers
The compressed header formats are defined as two separate sets for
each profile: one set for the headers where the innermost IP header
contains a sequential IP-ID (either network byte order or byte-
swapped), and one set for the headers without sequential IP-ID
(either random, zero, or no IP-ID). There are also a number of
common header formats shared between both sets. In the description
below, the naming convention used for header formats that belong to
the sequential set is to include "seq" in the name of the format,
while similarly "rnd" is used for those that belong to the non-
sequential set.
The design of the header formats is derived from the field behavior analysis found in Appendix A. All of the compressed base headers transmit lsb-encoded MSN bits and a CRC. The following header formats exist for all profiles defined in this document, and are common to both the sequential and the random header format sets: o co_common: This format can be used to update the context when the established change pattern of a dynamic field changes, for any of the dynamic fields. However, not all dynamic fields are updated by conveying their uncompressed value; some fields can only be transmitted using a compressed representation. This format is especially useful when a rarely changing field needs to be updated. This format contains a set of flags to indicate what fields are present in the header, and its size can vary accordingly. This format is protected by a 7-bit CRC. It can update control fields, and it thus also carries a 3-bit CRC to protect those fields. This format is similar in purpose to the UOR-2-extension 3 format of [RFC3095]. o co_repair: This format can be used to update the context of all the dynamic fields by conveying their uncompressed value. This is especially useful when context damage is assumed (e.g., from the reception of a NACK) and a context repair is performed. This format is protected by a 7-bit CRC. It also carries a 3-bit CRC over the control fields that it can update. This format is similar in purpose to the IR-DYN format of [RFC3095] when performing context repairs. o pt_0_crc3: This format conveys only the MSN; it can therefore only update the MSN and fields that are derived from the MSN, such as IP-ID and the RTP Timestamp (for applicable profiles). It is protected by a 3-bit CRC. This format is equivalent to the UO-0 header format in [RFC3095]. o pt_0_crc7: This format has the same properties as pt_0_crc3, but is instead protected by a 7-bit CRC and contains a larger amount of lsb-encoded MSN bits. This format is useful in environments where a high amount of reordering or a high-residual error rate can occur.
The following header format descriptions apply to profiles 0x0101 and
0x0107.
o pt_1_rnd: This format can convey changes to the MSN and to the RTP
Marker bit, and it can update the RTP timestamp using scaled
timestamp encoding. It is protected by a 3-bit CRC. It is
similar in purpose to the UO-1 format in [RFC3095].
o pt_1_seq_id: This format can convey changes to the MSN and to the
IP-ID. It is protected by a 3-bit CRC. It is similar in purpose
to the UO-1-ID format in [RFC3095].
o pt_1_seq_ts: This format can convey changes to the MSN and to the
RTP Marker bit, and it can update the RTP Timestamp using scaled
timestamp encoding. It is protected by a 3-bit CRC. It is
similar in purpose to the UO-1-TS format in [RFC3095].
o pt_2_rnd: This format can convey changes to the MSN, to the RTP
Marker bit, and to the RTP Timestamp. It is protected by a 7-bit
CRC. It is similar in purpose to the UOR-2 format in [RFC3095].
o pt_2_seq_id: This format can convey changes to the MSN and to the
IP-ID. It is protected by a 7-bit CRC. It is similar in purpose
to the UO-2-ID format in [RFC3095].
o pt_2_seq_ts: This format can convey changes to the MSN, to the RTP
Marker bit and it can update the RTP Timestamp using scaled
timestamp encoding. It is protected by a 7-bit CRC. It is
similar in purpose to the UO-2-TS format in [RFC3095].
o pt_2_seq_both: This format can convey changes to both the RTP
Timestamp and the IP-ID, in addition to the MSN and to the Marker
bit. It is protected by a 7-bit CRC. It is similar in purpose to
the UOR-2-ID extension 1 format in [RFC3095].
The following header format descriptions apply to profiles 0x0102,
0x0103, 0x0104, and 0x0108.
o pt_1_seq_id: This format can convey changes to the MSN and to the
IP-ID. It is protected by a 7-bit CRC. It is similar in purpose
to the UO-1-ID format in [RFC3095].
o pt_2_seq_id: This format can convey changes to the MSN and to the
IP-ID. It is protected by a 7-bit CRC. It is similar in purpose
to the UO-2-ID format in [RFC3095].
6.8.2.2. co_repair Header Format
The ROHCv2 co_repair header has the following format: 0 1 2 3 4 5 6 7 --- --- --- --- --- --- --- --- : Add-CID octet : if for small CIDs and CID 1-15 +---+---+---+---+---+---+---+---+ | 1 1 1 1 1 0 1 1 | discriminator +---+---+---+---+---+---+---+---+ : : / 0, 1, or 2 octets of CID / 1-2 octets if large CIDs : : +---+---+---+---+---+---+---+---+ |r1 | CRC-7 | +---+---+---+---+---+---+---+---+ | r2 | CRC-3 | +---+---+---+---+---+---+---+---+ | | / Dynamic chain / variable length | | - - - - - - - - - - - - - - - - r1: MUST be set to zero; otherwise, the decompressor MUST discard the packet. CRC-7: A 7-bit CRC over the entire uncompressed header, computed using the crc7 (data_value, data_length) encoding method defined in Section 6.8.2.4, where data_value corresponds to the entire uncompressed header chain and where data_length corresponds to the length of this header chain. r2: MUST be set to zero; otherwise, the decompressor MUST discard the packet. CRC-3: Encoded using the control_crc3_encoding method defined in Section 6.6.11. Dynamic chain: See Section 6.5.6.8.2.3. General CO Header Format
The CO header format communicates irregularities in the packet header. All CO formats carry a CRC and can update the context. All CO header formats use the general format defined in this section, with the exception of the co_repair format, which is defined in Section 6.8.2.2.
The general format for a compressed header is as follows:
0 1 2 3 4 5 6 7
--- --- --- --- --- --- --- ---
: Add-CID octet : if for small CIDs and CID 1-15
+---+---+---+---+---+---+---+---+
| first octet of base header | (with type indication)
+---+---+---+---+---+---+---+---+
: :
/ 0, 1, or 2 octets of CID / 1-2 octets if large CIDs
: :
+---+---+---+---+---+---+---+---+
/ remainder of base header / variable length
+---+---+---+---+---+---+---+---+
: :
/ Irregular Chain / variable length
: :
--- --- --- --- --- --- --- ---
The base header in the figure above is the compressed representation
of the innermost IP header and other header(s), if any, in the
uncompressed packet. The base header formats are defined in
Section 6.8.2.4. In the formal description of the header formats,
the base header for each profile is labeled
<profile_name>_baseheader, where <profile_name> is defined in the
following table:
+------------------+----------------+
| Profile number | profile_name |
+------------------+----------------+
| 0x0101 | rtp |
| 0x0102 | udp |
| 0x0103 | esp |
| 0x0104 | ip |
| 0x0107 | udplite_rtp |
| 0x0108 | udplite |
+------------------+----------------+
(page 45 continued on part 3)
